'Rcpp' Meets 'C++' Arrays.
Rcpp
interoperability with std::array
(et al.)
Rcpp
is a mature and very widely used package providing seamless interoperability between C++ and the R language, which has a native C API. This package facilitates conversion between R data structures and the templated std::array
class introduced in C++11, by using the interface provided in Rcpp
. It originated with this Stack Overflow question.
Support for std::tuple
was added in package version 0.3.0. This data structure, also introduced in C++11, combines a fixed set of elements of potentially different types.
std::array
is a templated container type with a fixed number of elements, an object-orientated analogue of a C-style array type like int[3]
. Client packages can interface this type with R if they add RcppArray
to LinkingTo
and include the header. (A simple example package is provided.)
// No need to include "Rcpp.h" as well
#include "RcppArray.h"
// [[Rcpp::export]]
RObject test() {
Rcpp::NumericVector vec = Rcpp::NumericVector::create(1,2,3);
std::array<double,3> arr = Rcpp::as<std::array<double,3>>(vec);
// Do something with the array
std::array<unsigned int,3> result = { 1, 2, 3 };
return result; // Implicitly Rcpp::wrap(result)
}
In either direction the element type of the array can be any atomic type that Rcpp
knows how to convert, and isn't limited to the types that R uses internally. In the example above, unsigned int
is used even though it doesn't directly correspond to an R vector mode: the wrap()
function will convert it to an R numeric
vector, i.e., double
.
A tuple, declared in C++ via a variadic template, as in std::tuple<int, double, std::string>
, can be converted similarly using Rcpp::as()
and Rcpp::wrap()
. In this case the R analogue is a list rather than an atomic vector, since the element types can vary.
std::span
There is also experimental support for std::span
, introduced in C++20. This is a typed container of fixed or dynamic length which provides a "view" onto a contiguous block of data owned by another object. Once again, there is a simple example client package showing usage of this facility.
In this case there are some notable caveats. Client packages must obviously request C++20 support, and a configure
script may well be needed to check for span
availability. Moreover, since a span
does not own the memory it points to, as<span<T,D>>()
will only compile where the requested type T
matches a type that R uses internally (viz. int
, double
, etc.). The latter limitation does not apply to wrap
, however, because a new vector is created and the data converted.